Choose the most suitable option to replace the highlighted part of the sentence:

I suggested that he goes to the veterinary surgeon immediately as his dog was not well.

The correct option to replace the highlighted part of the sentence is go to the veterinary surgeon immediately.

Explanation:
The sentence uses the verb suggested, which is a verb of suggestion or recommendation (such as suggest, recommend, demand, insist, propose). In English grammar, verbs of this kind require the main clause to be followed by a clause in the subjunctive mood.

In the subjunctive mood:
1. The base form (bare infinitive) of the verb is used regardless of the subject (e.g., he go instead of he goes).
2. Therefore, "that he goes" is grammatically incorrect in standard English and must be replaced with "that he go".

Comparing the given options:
- go to veterinary surgeon at once: Missing the definite article "the" before "veterinary surgeon".
- should goes to the veterinary surgeon: Incorrect verb form after the modal verb "should".
- go to the veterinary surgeon immediately: Uses the correct subjunctive base form "go" along with proper article usage and adverb placement.
- should have gone to the veterinary surgeon: Changes the intended meaning and time aspect unnecessarily.

Thus, the correct and grammatically proper replacement is go to the veterinary surgeon immediately.